Bug 1660213 - Fixed an used variable error in nsDocumentViewer::GetCurrentSheetFrameAndPageNumber. r=heycam

Differential Revision: https://phabricator.services.mozilla.com/D87806

layout/base/nsDocumentViewer.cpp

@@ -3432,6 +3432,7 @@ nsDocumentViewer::GetCurrentSheetFrameAndPageNumber() const {
 
   // in PP mPrtPreview->mPrintObject->mSeqFrame is null
   auto [seqFrame, pageCount] = mPrintJob->GetSeqFrameAndCountPages();
+  Unused << pageCount;
   if (!seqFrame) {
     return {nullptr, 0};
   }